Active Noise Control (ANC) systems attenuate undesired noise using feedforward and feedback structures to adaptively remove undesired noise within a listening environment, such as within a vehicle cabin. ANC systems generally cancel or reduce unwanted noise by generating cancellation sound waves to destructively interfere with the unwanted audible noise. Destructive interference results when noise and “anti-noise,” which is largely identical in magnitude but opposite in phase to the noise, combine to reduce the sound pressure level (SPL) at a location. In a vehicle cabin listening environment, potential sources of undesired noise come from the engine, the interaction between the vehicle's tires and a road surface on which the vehicle is traveling, and/or sound radiated by the vibration of other parts of the vehicle. Therefore, unwanted noise varies with the speed, road conditions, and operating states of the vehicle.
A Road Noise Cancellation (RNC) system is a specific ANC system implemented on a vehicle in order to minimize undesirable road noise inside the vehicle cabin. RNC systems use vibration sensors to sense road induced vibrations generated from the tire and road interface that leads to unwanted audible road noise. This unwanted road noise inside the cabin is then cancelled, or reduced in level, by using speakers to generate sound waves that are ideally opposite in phase and identical in magnitude to the noise to be reduced at the typical location of one or more listeners' ears. Cancelling such road noise results in a more pleasurable ride for vehicle passengers, and it enables vehicle manufacturers to use lightweight materials, thereby decreasing energy consumption and reducing emissions.
An Engine Order Cancellation (EOC) system is a specific ANC system implemented on a vehicle in order to minimize undesirable vehicle interior noise originating from the narrowband acoustic and vibrational emissions from the vehicle engine and exhaust system. EOC systems use a non-acoustic signal, such as a revolutions-per-minute (RPM) sensor, that generates a reference signal representative of the engine speed as a reference. This reference signal is used to generate sound waves that are opposite in phase to the engine noise audible in the vehicle interior. Because EOC systems use data from an RPM sensor, they do not require vibrations sensors.
RNC systems are typically designed to cancel broadband signals, while EOC systems are designed and optimized to cancel narrowband signals, such as individual engine orders. ANC systems within a vehicle may provide both RNC and EOC technology. Such vehicle-based ANC systems are typically Least Mean Square (LMS) adaptive feed-forward systems that continuously adapt W-filters based on both noise inputs (e.g., acceleration inputs from the vibration sensors in an RNC system) and signals of error microphones located in various positions inside the vehicle's cabin. ANC systems are susceptible to instability or divergence of the adaptive W-filters. As the W-filters are adapted by the LMS system, one or more of the W-filters may diverge, rather than converge to minimize the pressure at the location of an error microphone. Divergence of the adaptive filters may lead to broad or narrowband noise boosting or other undesirable behavior of the ANC system.